Is there anything to be learnt from casting techniques? Arrange a large sacrificial reservoir close to the rails by widening the bridge locally but which can be cut away afterwards? However, I suspect that the shrinkage happens as the epoxy starts to gel, by which time it is not going to flow anyway. And what's happened to the "negligible shrinkage" property that we're told epoxy has?

I have a vested interest in this as I shall be putting epoxy on my own rails shortly...